1. Filed of the Invention
The present invention relates to an organic light emitting device having an anode, a cathode, and a layer containing organic compounds in which luminescence can be obtained by applying an electronic field (hereafter referred to as organic compound layer). The present invention particularly relates to an organic light emitting element having a longer drive life than that of conventional ones, and a light emitting device using the organic light emitting element.
2. Description of the Related Art
An organic light emitting element is an element which emits light when an electronic field is applied. A luminescence mechanism thereof has been said: applying a voltage to an organic compound layer interposed between electrodes, electrons injected from a cathode and positive holes injected from an anode recombine together at a center of luminescence in the organic compound layer to form excited molecules; energy is released and light is emitted when the molecule excitons returne to the base state.
In addition, kinds of molecule excitons formed by the organic compound can include a singlet excited state and a triplet excited state, while the specification of the present invention contains the case where either of the excited states contributes to luminescence.
In such organic light emitting device, an organic compound layer is normally formed in a thin film below 1 xcexcm in thickness. Also, since the organic light emitting device is a self-luminescent type one, in which the organic compound layer itself emits light, a backlight used in a conventional liquid crystal display is not necessary. Accordingly, the organic light emitting device can be very advantageously formed to be thin and lightweight.
Also, with, for example, an organic compound layer of about 100 to 200 nm in thickness, a time period having elapsed from injecting of a carrier to recombination thereof is in the order of several tens of nanosecond taking account of the extent of movement of the carrier in the organic compound layer, and luminescence is achieved in the order of less than one micro second even when the procedure from the recombination of the carrier to luminescence is included. Accordingly, one of the features is that the speed of response is very quick.
Further, since the organic light emitting device is a carrier-injecting type light emitting device, it can be driven by DC voltage, and is hard to generate noise. With respect to drive voltage, an adequate luminance of 100 cd/m2 is achieved at 5.5 V by first making the thickness of an organic compound layer a uniform, super-thin film of around 100 nm in thickness, selecting an electrode material, which reduces a carrier injection barrier with respect to the organic compound layer, and further introducing a single hetero structure (double structure) (Reference 1: C. W. Tang and S. A. VanSlyke, xe2x80x9cOrganic electroluminescent diodesxe2x80x9d, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)).
Owing to such performances as thin and lightweight, high-speed response, DC low voltage drive, and the like, organic light emitting devices have been given attention as next-generation flat panel display devices. Also, since organic light emitting devices are of self-luminescent type and wide in angle of visibility, they are comparatively favorable in visibility and believed to be effective as devices used for displays in portable devices.
However, the reliability of the device can be a major problem of such a light emitting device. With respect to the reliability, the luminance being deteriorated over time is a particularly conspicuous problem, and a considerable improvement is needed. Further, being a super thin film device, defects such as short circuits of the device caused by the morphology, point defect or unevenness of the thin film must be prevented.
It can be considered that the luminance being deteriorated over time is a phenomenon basically caused by the materials, however, it is possible to prolong the half-life of luminance by driving methods of the device. For example, there is an example that the half-life of luminance being deteriorated over time has been improved dramatically by inserting copper phthalocyanine as a hole injecting layer, and employing a rectangular wave alternating current drive (constant currents in forward biases, constant voltages in reverse biases) instead of a direct current drive (Reference 2: S. A. Van Slyke, C. H. Chen, and C. W. Tang, xe2x80x9cOrganic Electroluminescent Devices with Improved Stabilityxe2x80x9d, Appl. Phys. Lett., 69 (15), 2160-2162 (1996)).
According to reference 2, the luminance half-life at initial luminance 510 cd/m2 can be prolonged to 4000 hours. As factors thereof, an excellent positive hole injecting characteristic of copper phthalocyanine which is a positive hole transportation layer, a superior heat-resistance of NPB which is a positive hole transportation layer, and a capability of excluding the accumulation of space charges by an alternating current drive can be given. In addition, normally, when the organic light emitting element is driven by a constant current, a drive voltage rises gradually along with the luminance deterioration, however, the rises of the drive voltage can be suppressed by the alternating current drive.
It has been reported that defects such as short circuits of the device caused by the morphology and point defects can be removed by such alternating current drive (Reference 3: Japanese Patent Laid-Open No. 08-180972). Reference 3 has pointed that the short circuits present frequently in a short period (200 to 300 hours) in the case of direct current drive as a result of comparing the lifetimes of the alternating current drive and the direct current drive.
Further, as techniques for preventing the short circuits of the element caused by unevenness of an electrode, a technique such as providing a buffer layer comprising a conductive polymer on the electrode is devised (Reference 4: Yoshiharu Sato, Organic Molecules and Bioelectronics (The Japan Society of Applied Physics, Vol. 11, No. 1 (2000), p. 86-99). Reference 4 has mentioned that by introducing an anode buffer layer of polymer, the surface unevenness of ITO which is an anode can be smoothed, thereby leads to the reduction of the defects of short circuits.
As described above, in order to improve the reliability of the organic light emitting element, techniques can be implemented not only from the viewpoint of improvements of materials, but from the viewpoint of the drive methods and the structure of the element.
The alternating current drive described in the document 3 is characterized in that the voltage applied to the device in reverse bias (hereafter, it is denoted by Vr) is equal to or greater than the voltage applied to the device in forward bias (hereafter, it is denoted by Vf) and the time for duration in reverse bias (hereafter, it is denoted by Tr) is shorter than the time for duration in forward bias (hereafter, it is denoted by Tf). More specifically, Vfxe2x89xa6Vr and Tf greater than Tr. Here, the voltages shown here are all handled as positive values.
Considering the effect that the reverse bias eliminates the accumulation of space charges, Vfxe2x89xa6Vr is more effective. However, in this case, it is necessary to take into account of the dielectric breakdown strength of the element. In short, Vr has to be set smaller than the dielectric breakdown voltage Vb. That is, Vfxe2x89xa6Vr less than Vb.
However, when the luminance is being reduced over time, the current efficiency (the ratio of the luminance to the current density) itself is being reduced compared with the initial driving period. In other words, since the leakage current not contributing to light emission is increased, it is likely to destroy the element by dielectric breakdown with the voltage smaller than Vb in the long term.
That is, in the case of the alternating current drive method, the half-life of luminance is prolonged more than that by direct current, and short circuits generated in the initial driving period can be prevented. However, in the long view, the possibility of dielectric breakdown emerges even in the reverse bias voltage Vr where the element was not supposed to be destroyed by dielectric breakdown in the initial driving period.
Then, the object of the invention is to improve the alternating current drive method of the organic light emitting element capable of relaxing the luminance deterioration and preventing short circuits in the initial driving period more than before and to prevent the device from being destroyed by dielectric breakdown in a long term. In addition, the object is to provide a light emitting device with reduced luminance deterioration and excellent yields by combining the alternating current drive means and the organic light emitting element.
Moreover, the object is to provide an electric device excellent in long time reliability more than before by manufacturing the electric device with the use of the light emitting device.
The invention is characterized by a light emitting device having:
an organic light emitting element provided with an organic compound layer for obtaining light emission by applying a voltage between a first electrode and a second electrode; and
means for applying a forward bias for emitting light from the organic light emitting device and a reverse bias of reverse polarity to the forward bias,
wherein a maximum voltage Vf of the forward bias is greater than a maximum voltage Vr of the reverse bias in an alternating current cycle formed of the forward bias and the reverse bias. At this time, the Vr is preferably a quarter of the Vf or greater.
In addition, a period of time to apply the reverse bias in the alternating current cycle is preferably equal to or longer than a period of time to apply the forward bias. In this case, an amount of light emission becomes small and a period of time for light emission is shortened. Thus, the organic light emitting device for use preferably produces light emission from the triplet excited state. Alternatively, light emission is preferably produced from a rare-earth metal ion.
Here, current is hardly carried in reverse bias, and thus it is acceptable to apply a certain fixed voltage for the reverse bias. As for the forward bias, a fixed voltage is acceptable, but the drive to carry a constant current is more preferable.
Accordingly, the invention is characterized in that both the forward bias and the reverse bias are applied so that a fixed voltage is applied to the organic light emitting element. Furthermore, the invention is characterized in that the forward bias is applied so that a fixed current is carried through the organic light emitting device and the reverse bias is applied so that a fixed voltage is applied to the organic light emitting element.
In the meantime, as the organic compound layer in the invention, a polymer compound is preferably used. Then, the invention is characterized in that the organic compound layer contains a polymer compound for producing light emission.
Moreover, in the invention, a layer having a certain level of conductivity is preferably disposed between the organic compound layer and the electrode.
Accordingly, the invention is characterized in that a conductive layer containing an inorganic compound is disposed between the organic compound layer and the first electrode, or between the organic compound layer and the second electrode. In this case, the conductivity of the conductive layer containing the inorganic compound is preferably 10xe2x88x9210 S/cm or greater.
Besides, the invention is characterized in that a layer containing a conductive polymer compound is disposed between the organic compound layer and the first electrode, or between the organic compound layer and the second electrode. Particularly, in view of improving the conductivity, the layer containing the conductive polymer compound preferably further has an acceptor or a donor for the conductive polymer. Also in this case, the conductivity of the layer containing the conductive polymer compound is preferably 10xe2x88x9210 S/cm or greater.
In addition, the invention is characterized in that a layer containing a pi-conjugated organic compound is disposed between the organic compound layer and the first electrode, or between the organic compound layer and the second electrode. Particularly, in view of improving the conductivity, the layer containing the Π-conjugated organic compound preferably further has an acceptor or a donor for the Π-conjugated organic compound. Also in this case, the conductivity of the layer containing the Π-conjugated organic compound is preferably 10xe2x88x9210 S/cm or greater.
Therefore, the light emitting element described above is used to manufacture an electric device, which can provide an electric device more durable than before. Accordingly, the invention includes an electric device using the light emitting element according to the invention.
The light emitting device in the invention is an image display device or a light emitting device using the organic light emitting element as a light emitting element. In addition, a module in which an organic light emitting element is mounted with a connector, such as a FPC (Flexible Printed Circuit), TAB (Tape Automated Bonding) tape or TCP (Tape Carrier Package), a module in which a printed wiring board is mounted on the tip of a TAB tape or a TCP, or a module in which an organic light emitting element is directly mounted with an IC (Integrated Circuit) by COG (Chip On Glass) method is all considered to be included in the light emitting device.